May June 1980 - Commodore Computers

MAY/JUNE. 1980. 1CW 104 COMPUTE. I 1960. ISSUE 4.
Block Access Method
Block Access Method
Map for a Commodore
2O4O 2040 Disk Drive
Map for a
San Jose, Calif. CommodoreanJOSe, f. 95136
OVERVIEW
OVERVIEW
The Block Access Method (BAM) map program will
allow you to see where your files arc are allocated. You
can save and delete files and observe the allocation
technique.
DESCRIPTION
The purpose of the BAM is to protect allocated
files so they are not written over and therefore
destroyed. The BAM map resides on the directory
track 18. The BAM is in the first half of sector 0. O.
The layout looks like this:
BAM Dump
TRACK 18 SECTO R 0
TRACK 18 SECTOR 0
Track
a
b
NUMBER
00 :/112011012 0011 00|l5FFFFlFl
5 I FI
08 : IS 15FFFF1F
FF I F IS 15FFFF1F
IF
10: 15 15FFFF1F FFFF IF 15FFFF1F
IF
18: 15FFFFIFI5 15FFFF1F 15FFFF1F FFFFIF
20: IS 15FFFF FF FF I IF F IS 15FFFF1F FF FF IF
28 : IS 15FFFF1F FF FF I F IS 15FFFF1F FF FF IF
30 : IS 15FFFF1F FF I F IS 15FFFF1F FF FF IF
38 : IS 15FFFF1F FF FF I F IS 15FFFF1F
IF
40: 15 15FFFF1F FFFF IF IS 15FFFF1F
IF
48 : 12 FC FF OF 14 FF FF OF
48 12FCFF0F 14FFFF0F
50 : 14FFFFOF14FFFF0F
58 : 14 14FFFF0F14FFFFOF
60
60
: 14 FF FF OF .............
12 FF FF 03
12FFFFO3
68: 12 12FFFF03 FF03 12 12FFFF03
FF03
70 70: : 12 12FFFFO3 03 12 12FFFFO3
03
78 78: : 12 12FFFF03 FF FF03 ..........._—
II FFFFOI
2 3
4 5
6 7
8 9
10 II
10 11
12 13
14155
1617
16 17
18 19
18 19
2021
21
2223
23
24
25
26 27
28 29
30
11FFFF01 31
80: : II 11 FFFF01 OIII 11 FFFF01 OI
3233
33
82 82: : I11I FFFF01 0 I II 11FFFF01 0 I
3435
35
a •- Address of the next sector which is where the directory begins.
b .- The start of the BAM map for trackk I. 1.
24
30
25
31
Detail Explanation
a b c d
00: : 1201 1 01 0 1 00ll5|FF|FFllF|
001151 FFI FFII FI
a - Total free sectors for track 1. I. In this case it is hex
15 or decimal 21. 2 Since track 1 has a maximum of 21
sectors, track 1 is totally (Otall y empty.
b - The bit configurations for sectors 0 thru Ihru 7. Bit on
means empty sector scc lO r and bit off orf means allocated sector.
hex FF = bits" "1111 III 1111"
"
I Oth Ath sector
lth 1 sector
2th sector
r
3th sector
lOr
4th sector
5th sector
6th sector
7th sector
hex FF = bits ""1111 1111" "
Detail Explanation
7th sector
Therefore all sectors are empty.
c - bit configurations for sectors 8 thru 15
I ~th
8th sector
9th sector
Oth sector
11 llth
sector
12th sector
13th sector
4th sector
15th sector
d - bit configurations for sectors 16 thru 20.
hex IF = bits ""0001 1)1"
1111"
16th sector
7th sector
18th sector
19th sector
20th sector
71 6T?~aq~~aa~
secto r
15th sector
d - bit configurat ions for sectors 16 thru 20.
16th sector
20th sector
In a ny empty disk, the ' d ' byte changes from 1 F, OF , 03, 01
In any empty disk, the 'd' byte changes from IF, OF, 03, 01
to compensate for varying number of sectors per track.
track,
hex 1 IFF is the pattern where there arc 211 sectors
as in tracks 1 thru 17. .
hex OF is the pattern where there are 20 sectors as
in tracks 18 thru 24.
hex I IFF is the pattern where there are arc 18 sectors as in
tracks 25 thru 30. .
hex 1 IFF is the pattern where there are 17 sectors as in
tracks 3311 thru 35.